Over the years many pressure-relief structures have been incorporated in the design of hermetically sealed electrolytic capacitors, often taking the form of valves. The can is sealed to preserve the electrolyte against drying out or leaking. Explosive pressure can thus develop inside the can due to malfunction or misapplication. Pressure-relief structures devised for large capacitors have been found impractical for incorporation into capacitors having small-size cans, e.g. about 1/2-inch diameter and less. In the absence of a distinct pressure-relief device, it has been known that the entire end-seal structure would be released suddenly upon development of internal pressures approaching explosive levels and even at much lower pressures, representing a safety hazard. The entire capacitor electrode assembly might then be projected violently and dangerously from the can.
As one solution to this problem for special application to small-diameter cans, it has been proposed that a weakening incision in the end or the side wall of the can would provide safe pressure-relief. That approach, while effective, has required the exercise of great care in achieving precisely the right amount of penetration of the incision into the wall of the can. Despite care in making the incisions, the pressure level at which venting occurs is variable. If the incision is too deep, the can vents too readily and the capacitor fails prematurely. If the incision is not deep enough, the pressure may well blow out the end seal before the incision function. The incision cannot be tested to sort out the capacitors having the proper venting range since the test is destructive. Moreover, the incision represents a weak point that could develop into a leak. The incision can also be breached accidentally in handling, as by a fingernail, particularly where the incision is made on the outside surface of the can.